Heated flow and liquid level sensors have been available for at least several decades. Examples of such devices and the manner in which they function are shown in U.S. Pat. Nos. 3,366,942, 3,898,638, 4,899,584, 4,967,593, and 5,600,528.
Where safety is of high concern, an extremely low failure rate for such sensors is a goal to strive for. More specifically, reliability requirements that are greater than one failure per million (>10−6) hours of operation are not uncommon.
For example, in aircraft the term “flight critical” is applicable. In certain instances, a failure indication, or an indication of a function that is diminishing below normal and approaching possible failure, warns the aircraft personnel that as landing is either urgent (within 20 minutes, for example) or that safe flying has a measured time, one hour for example, and that a location for a safe landing should be determined within that time. In such operating conditions, false positives cannot be tolerated. An indication of an important failure, if due to sensor failure, could require an aircraft to needlessly land, or to shut down major machinery or a processing plant.
To reduce false positives to acceptable levels, that is, close to zero, sensor redundancy has been used. However, in aircraft, again as an example, a redundant sensor assembly may well not be an acceptable alternative. Such a limitation may be due to size or weight limitations, or the fact that no space or openings are possible far a second sensor assembly.
An example of a prior art fluid flow or liquid level sensor is shown in FIG. 1. The sensor head assembly has two thermowells 2 and 3 configured to extend into the fluid to be sensed. One of the thermowells is an active sensor and the other is a reference sensor. As stated above, to achieve redundancy, two such sensor head assemblies would need to be mounted through the wall of a conduit in reasonably close proximity so that two separate signals can be processed and compared. For the reasons stated, such dual installations are not practical, or even possible, in many cases.